Non-Adiabatic Photoprocesses of Fundamental Importance to Chemistry: From Electronic Relaxation of DNA Bases to Intramolecular Charge Transfer in Electron Donor-Acceptor Molecules

Abstract

Substituent effects on ultrafast electronic relaxation (internal conversion) of nucleobases and intramolecular charge transfer in electron donor-acceptor (EDA) molecules, containing benzonitrile and diphenylacetylene moieties, have been investigated using laser spectroscopy and simple ab initio methods. The results demonstrate the central role biradical states play in the nonadiabatic energy- and charge-transfer dynamics. Specifically, subpicosecond internal conversion characteristic of the naturally occurring nucleobases is effectively extinguished by covalent modification that prevents the out-of-plane deformation of the π π* singlet state into the twisted ethene-like structure of the biradical state. Similarly, the covalently modified EDA molecules, in which the biradical (π σ*) state of bent geometry lies substantially above the optically prepared ππ* state, do not exhibit intramolecular charge transfer even in highly polar solvents.